One of the problems facing users of hand-held vibrating equipment is exposure to elevated mechanical vibration levels. Long term exposure has produced symptoms of vascular, nervous system and bone/muscle deterioration such as hand-arm vibration syndrome and white hand. Many have attempted to solve the problem of excessive vibration transmitted to the users of hand-held tools by incorporating elastomer elements between the user and the vibrating device. Approaches have attempted to isolate and/or damp the mechanical vibration of the device.
One such isolating approach is taught in U.S. Pat. No. 3,968,843 to Shotwell, which is hereby incorporated by reference, and provides a pneumatic air hammer with a shock and vibration-absorbing insert or cushion member 30 between the body of the tool 10 and the handle 19. The isolator used is a plain compression-type sandwich isolator. Its theory of operation is to place a soft spring between the user and the vibrating device, thus isolating the user from mechanical vibration. However, compression-type isolators have one serious drawback. They experience an inherent stiffening effect when the operator exerts an increased force on the tool. This is due to the inherent strain sensitivity of elastomer in compression. Because of this, as the force increases, the level of vibration felt by the user is worsened. In other words, the harder the operator pushes the more ineffective the isolator becomes.
In addition, in order to maintain control of the tool, the cocking and torsional motions of the tool must be restrained. U.S. Pat. No. 2,500,036 to Horvath uses dual resilient members 80 and 81 to allow limited axial movement and restrain cocking. It also uses a plurality of locking segments 85 to restrain torsional rotation of the handle member 13 relative to the barrel 10.
In U.S. Pat. No. 5,054,562 to Honsa et al., an isolator which was to provide axial vibration isolation as well as cocking/torsional control by surrounding the working cup 20 with laminar layers of elastomer is described. Although this makes for a convenient package, this has the same inherent problem of compression strain stiffening as the Shotwell '843 approach.
As taught in U.S. Pat. No. 4,401,167 to Sekizawa et al., others have attempted to place the elastomer elements 6a and 6b between the tool body 1 and the handle 2. Although placing the elastomer in shear substantially eliminates the strain stiffening effects, it cannot provide low enough stiffness for optimum control and still maintain control of the tool.
Further attempts to improve the vibration isolation characteristics of hand-held tools have included the addition of fluid damping to the isolator. By adding damping, over and above what is available from an elastomeric device alone, the vibrations emanating from the tool can be further reduced. U.S. Pat. No. 4,667,749 to Keller, which is hereby incorporated by reference, describes such an isolator which adds fluid damping to an isolator and which is suitable for mounting a handle to a vibrating tool body.
Further, U.S. Pat. No. 4,236,607 to Hawles et al. describes a vibration suppression system wherein the fluid passes through the inner member of the mounting to provide amplified counter inertial forces. The commonly assigned U.S. Pat. No. 4,969,632 issued to Hodgson et al. and U.S. Pat. No. 4,733,758 issued to Duclos et al., which are both hereby incorporated by reference, describe other tunable mountings.